Process control plants or systems often employ rotary valves, such as ball valves, butterfly valves, eccentric-disk valves, eccentric-plug valves, etc., to control the flow of process fluids. Rotary valves typically include a fluid flow control member (e.g., a disc, a ball, etc.) within a flow control aperture of the valve and rotatably coupled to the body of the valve via a shaft. To control the flow of fluid through some rotary valves, the position of the fluid flow control member may be varied from a closed position at which the fluid flow control member is in sealing engagement with a seal that surrounds the flow control aperture to a fully open or maximum flow rate position at which the fluid flow control member is spaced away from the seal.
The structure of a valve may be varied to suit the needs of a particular process control environment. For example, a butterfly valve may be a double offset valve (also known as a high-performance butterfly valve) or a triple offset valve. Triple offset valves are similar in design to double offset valves except that the control member has an eccentrically shaped circumferential surface to sealingly engage the valve seal. The angled geometry of the control member of a triple offset valve reduces contact area between the control member and the seal during closure, thus reducing wear in any application (e.g., throttling or on-off). However, triple offset valves are more expensive and can be disadvantageous due to the high torque required to engage the control member and the seal to achieve a tight shutoff.
The material selected for a seal may vary depending on the requirements of the process control environment in which the seal is to be used. For example, seals may be made of metal, polytetraflouroethylene (PTFE), or laminated graphite. A metal seal is well suited for use with high temperature and high pressure process applications but is generally more susceptible to wear. A PTFE seal provides excellent sealing performance compared to metal seals and requires less torque to unseat a disc from the seal. PTFE seals also provide a relatively long seal life but are limited to process applications that maintain temperatures below 450 degrees Fahrenheit. A graphite laminated seal can withstand much higher temperatures than a PTFE seal and provides excellent shutoff and the ability to maintain tight shutoff after a large number of cycles.